Presses incorporating a brake and a clutch to control the operation of a crankshaft are well known in the art. Presses of this type generally have the crankshaft intermittently driven through the clutch from a continuously running flywheel, or gear. The crankshaft of the machine may be operated in several different ways, depending on the manner in which the press is being used and the type of work it is performing, but the mode of operation most generally employed is to intermittently drive the crankshaft through one cycle, stopping it at the same point after each cycle of movement. The operation of the crankshaft is controlled by the clutch which connects and disconnects the crankshaft and the flywheel as desired, and a brake which stops the movement of the crankshaft when the clutch is disengaged. The control of the clutch and brake are interconnected in such a manner that the brake will be released simultaneously with the engagement of the clutch and vice versa. The operation of the clutch and brake is controlled by a suitable hydraulic and/or electric apparatus which is operated manually or automatically to effect the engagement of the clutch and the release of the brake and is normally operated automatically to effect the disengagement of the clutch and the application of the brake.
Some designs for these presses utilize air pressure to activate dry-friction brakes and clutches for the purposes of cycling the press. One problem which has occurred in these dry-friction, air operated brakes and clutches is that there has been excessive trouble with the dry-friction brake while only minimal trouble with the dry-friction clutches. One of the reasons that the dry-friction clutches will perform quite well and the dry-friction brakes will not lies in the design of the clutch and the brake. Clutches are normally designed and sized to produce the rated tonnage of the press. Brakes, on the other hand, are designed and sized to stop the press. Thus, the clutches are normally designed to be significantly larger than the brakes. This means that the clutches have more disc friction area available to dissipate the heat caused by starting than the brakes have for stopping the press. Since the same press inertia is present in the clutch and the brake function, the same amount of heat needs to be dissipated when starting or stopping the machine. This leads to a significantly shorter life for the brakes than for the clutches.
Another problem in the clutch and brake design for these presses is the clutch and brake overlap. Even though the clutch and the brake are operated simultaneously, there are still times when the clutch starts to engage before the brake is released and vice versa. The clutch is normally engaged by air pressure and is disengaged by springs. The rated torque or full engagement of the clutch is typically reached with 70-80 psi of air pressure. Engagement of the clutch usually begins at approximately 10 psi of air pressure due to the air pressure beginning to overcome the load exerted by the springs. The brake is normally applied by springs and released by air pressure. The release of the brake typically begins with approximately 40 psi of air pressure due to the fact that the springs must exert sufficient load to apply the brake. This function is opposite for the clutch where the springs are only required to separate the plates for disengagement of the clutch. The time it takes for the air pressure to build from 10 psi to 40 psi on starting is the starting overlap. The time it takes to exhaust air from 40 psi to 10 psi on stopping is the stopping overlap. When the clutch torque reaches a point that it is significantly larger than the brake torque, the clutch will quickly overpower the brake on starting and overpower the brake on stopping providing the starting and stopping overlap. These overlaps cause overheating of the brake and rapid disc wear for the brake friction plates.
Accordingly, there is a need for a brake and clutch control system which is capable of eliminating the overlap between the clutch and the brake during both the starting and stopping of the machine. The control system should be as simple as possible, low cost and be capable of being retro fitted onto existing presses at the time of brake replacement.